Overview - it's free, it's a bass guitar synth, it doesn't claim to do anything else and it actually does its job reasonably well. It couldn't be any easier to program to tweak the sound, and although there are better ways of getting a bass guitar sound more accurately, it a useful tool to have. Got mine free with Cubase.
Sound - you can't use it for every occasion you need a synth bass guitar, but it definitely sounds OK for general purposes. I find myself using it more and more simply for the fact that it can be quite tedious to patch VSTi's to make a realistic bass guitar, and I'm not into samplers that much. I sometimes lay down my basslines with VB1 and then load up a different VSTi to get the fine-tuned sound I need, but other times if your not too fussy about the sound, VB1 will suffice more than adequately. As mentioned, it defaults to picked sounds, and I've never managed to completely get rid of the initial picking click - not always a bad thing, but when doing e.g. dub style music when you don't want that click, then it isn't the best tool. Especially if you're going to compress your bass guitar sound - it can be quite annoying as it really shows up the click. But if you do want a picked sound, then it's fine. It will also do a few more synthesized type sounds very reminiscent of an FM synth, so it's not just a one-trick pony - no amzing synth sounds, but enough to vary it up a bit.
CPU - uses barely any CPU, which is why I often use it to lay down ideas with before rendering to audio.
GUI - couldn't be any easier. A complete novice will be able to tweak the sounds with no difficulty whatsoever.
Stability - I've never had any problems with it at all. No glitches when tweaking parameters whilst playing.
Presets - It comes with a handful, and doesn't really need any more because it's so easy to use.
VFM - it's free. No point in not at least giving it a try.
Features - you can adjust the bridge position, the string height, the pickup position, tone etc - all you need really. I would prefer an option to completely switch off the picking click. I would probably use it much more often if it had this. I assume it's a physical modelling synth, but I don't know for sure - but whatever...it sounds close enough to a real bass guitar for general purpose use or for laying down ideas. Not useable in every situation, but I've heard far worse, and to be honest there are plenty of VSTis with the full set of editable parameters that struggle to achieve anything like a bass guitar sound, so you shouldn't write VB1 off. Bass is one of those areas that is surprisingly difficult to get right, and VB1 lets you get the basics right - I'd happily use it for anything middle-of-the-road or pop where it's not that important to get unique and original bass patches going. It doesn't pretend to be a Moog Modular and if you don't expect it to be, then it won't disappoint you too much.